(1) Field of the Invention
The present invention relates to a system and method for automatically controlling a vehicle speed to a desired cruising speed.
(2) Background of the Art
A Japanese Patent Application First Publication (non-examined) No. sho 47-35692 published on Nov. 25, 1972 exemplifies a system for automatically controlling a vehicle speed to a desired cruising speed through a control over an opening angle of an engine throttle valve.
In the above-identified Japanese Patent Application publication, an error .epsilon..sub.I between a target vehicle speed V.sub.s and actual vehicle speed V is derived in accordance with an equation expressed below for each predetermined control period (for example, 30 msec.). EQU .epsilon..sub.I =V.sub.s -V
In addition, a change rate error .epsilon..sub.p between an actual vehicle speed V.sub.B before one control period and the instantaneous vehicle speed V.sub.N is derived in accordance with an equation expressed below.
.epsilon..sub.p =V.sub.B -V.sub.N
An opening angle adjustment quantity .DELTA..theta. for an engine throttle valve is derived in accordance with an equation expressed below.
.DELTA..theta.=K.sub.I .times..epsilon..sub.I +K.sub.p .times..epsilon..sub.p,
wherein K.sub.I and K.sub.p are gain constants.
Therefore, the engine throttle valve is open or closed through an angular displacement indicated by an absolute value of .DELTA..theta. of the adjustment quantity of the opening angle in a direction denoted by .DELTA..theta.. This permits the adjustment of an engine torque, thus the vehicle cruising at the target vehicle speed.
The engine torque characteristic is such that the engine torque is saturated when the opening angle of the throttle valve indicates 40 degrees through 80 degrees with respect to a fully closed position. Therefore, if the throttle valve opens through the above-described angle area, almost no increase in the engine torque occurs and therefore little increase in the vehicle speed results.
Consequently, the throttle valve is widely opened more than necessary with no acceleration of a vehicle in a case where the vehicle cruises on an ascending slope with a transmission gear selected at a high gear position.
In a case where the vehicle cruises on a road from the ascending slope to a flat, a control delay occurs until the throttle valve which has been opened widely during the slope run is returned to an opening angle through which the vehicle can cruise on the flat road causing the vehicle speed to excessively deviate from the target vehicle speed.